The present invention relates generally to a method and apparatus for measuring and displaying pulse rate with increased accuracy. More specifically, the present invention provides a method for increasing the accuracy of a pulse rate sensing system by means of a novel pressure sensing array and multiple methods for identification and elimination of artifacts.
Other methods and apparatus are known for measuring pulse rates and for rejecting pulse artifacts. For example, U.S. Pat. No. 4,409,983 shows a pulse measuring device which employs multiple transducers connected to averaging circuits and differential amplifiers. This invention helps separate signals corresponding to motion artifacts from the signal corresponding to a heartbeat pulse. Other apparatus and methods for removing motion artifacts are disclosed in U.S. Pat. Nos. 4,307,728, 4,202,350, 4,667,680, 4,239,048, 4,181,134 and 4,456,959. Methods used to reduce signal errors include the use of windowing and averaging techniques and auto correlation algorithms.
The pulse rate sensor systems described above are subject to several sources of inaccuracies. First, it is difficult to reject motion and noise artifacts in many of these systems. This is especially true for systems employing a single sensor element. (See U.S. Pat. Nos. 4,202,350 and 4,239,048.) These systems have no physical means for receiving both a pulse-plus-artifact signal and a separate artifact signal. Other means are required to compensate for, or eliminate, the error caused by artifacts such as motion artifacts. Signal processing techniques such as filtering and windowing are often used.
Even those systems or methods which employ multiple sensor elements inaccurately measure pulse rate because only a single method is used for enhanced signal processing. For example, different types of motion artifacts can occur simultaneously, and with other pertubations, on the pulse sensor. It is also not unusual for signal errors to be interpreted as pulses or for actual pulses to be missed by the pulse sensor. Methods of pulse rate determination which do not compensate for these errors are inherently inaccurate under real-world conditions ere artifacts are present.
For example, if a pulse rate system detects a "pulse" caused by noise, several adverse results may be seen. The pulse rate system could use the noise as the basis for windowing the signal. The pulse rate system could simply use this "pulse" as part of the overall pulse rate calculation. In addition, the pulse rate system could recognize the noise as noise and subtract out the noise, in some cases subtracting out a valid signal as well.
Another source of inaccuracy that occurs using pulse measuring devices that measure pressure variations caused by a subject's pulse (see U.S. Pat. No. 4,409,983 for example) is inverted pulse waveforms. An inverted waveform can occur when the housing that holds the pressure sensitive element(s) is located on the artery, but the pressure sensitive element(s) itself is located off the artery. In this case the subject's pulse can push up on the housing and lessen the pressure on the pressure sensitive element. The result is that a pulse waveform is still received, but it is inverted and shows a negative relative pressure. Pulse measuring devices which rely on pressure measurements but can correctly interpret only positive pressure waveforms must be placed and held accurately on the artery, creating additional demands on attachment of the device and/or lowering comfort to the user.
Additionally, pronounced dichrotic notches can be found in the pulse of many people. When dichrotic notches are present there are two rises and two falls in blood pressure during a single heartbeat. These can be mistakenly interpreted as two heartbeats, leading to a major inaccuracy in pulse rate measurement.
The present invention overcomes the problems encountered with other pulse rate sensors by applying the principles of arterial tonometry for signal acquisition for a pulse rate sensor. In the invention, multiple algorithms are used in signal processing and pulse rate calculation to compensate for multiple signal errors which could occur during pulse rate measurement.
The principles of arterial tonometry are described in several U.S. Patents including: U.S. Pat. Nos. 3,219,035; 4,799,491 and 4,802,488. These principles are also described in several publications including an article entitled "Tonometry, Arterial," in Volume 4 of the Encyclopedia of Medical Devices and Instruments. (J. G. Webster, Editor, John Wiley & Sons, 1988). All of these references discuss arterial tonometry as used for the measurement of blood pressure.
For blood pressure measurement, it is desirable to flatten a section of the arterial wall as described in these references. Flattening is produced by exerting an appropriate hold down force on the tonometer sensor. For pulse sensing, significant flattening of the arterial wall is not necessary and a lower hold down force can be used. This results in greater comfort for the wearer.